Droplet ejection devices are used for a variety of purposes, most commonly for printing images on various media. They are often referred to as ink jets or ink jet printers. Drop-on-demand droplet ejection devices are used in many applications because of their flexibility and economy. Drop-on-demand devices eject one or more droplets in response to a specific signal, usually an electrical waveform that may include a single pulse or multiple pulses. Different portions of a multi-pulse waveform can be selectively activated to produce the droplets.
Droplet ejection devices typically include a fluid path from a fluid supply to a nozzle path. The nozzle path terminates in a nozzle opening from which droplets are ejected. Each ink jet has a natural frequency which is related to the inverse of the period of a sound wave propagating through the length of the ejector (or jet). The jet natural frequency can affect many aspects of jet performance. For example, the jet natural frequency typically affects the frequency response of the printhead. Typically, the jet velocity remains near a target velocity for a range of frequencies from substantially less than the natural frequency up to about 25% of the natural frequency of the jet. As the frequency increases beyond this range, the jet velocity begins to vary by increasing amounts. This variation is caused, in part, by residual pressures and flows from the previous drive pulse(s). These pressures and flows interact with the current drive pulse and can cause either constructive or destructive interference, which leads to the droplet firing either faster or slower than it would otherwise fire.
One prior ink jetting approach uses a pulse string followed by a cancelling pulse. The cancelling pulse is a shortened pulse that is timed so that the resulting pressure pulses arrive at the nozzle out of phase with the residual pressure from previous pulses. Given that jets will have a dominant resonant frequency, the cancellation features are timed in units of resonance period Tc. FIGS. 1a and 1b show two common types of cancellation pulses: same sense cancellation pulse 180 in FIG. 1a and opposite sense cancellation pulse 199 in FIG. 1b. A same sense cancellation pulse is preceded by a cancel edge delay, which has a voltage level that is similar to a voltage level of one or more delays between drive pulses. An opposite sense cancellation pulse is preceded by a cancel edge delay, which has a voltage level that is different than a voltage level of one or more delays between drive pulses. The voltage level of the cancel edge delay is in the opposite direction, relative to the bias level or level between fire pulses, compared to the fire pulse. FIG. 1a illustrates a fire edge of pulse 181 that is followed by a cancellation puke delay 182 (e.g., To) and then cancellation pulse 180. FIG. 1b illustrates pulses 190-197, a fire edge 198 that is followed by a cancellation pulse delay 184 (e.g., Tc) and then cancellation pulse 199. In these architectures, a large droplet is created by expressing all the pulses while smaller droplets are expressed by removing the earlier pulse(s). Hence, considering the opposite sense cancellation pulse 199 shown in FIG. 1b, a middle droplet may be constructed from pulses 193, 194, 195, 196, 197, and the cancellation pulse 199 while a small droplet could be formed using the pulse 197 and the cancellation pulse 199.
FIGS. 2a and 2b show prior waveform designs for a small droplet with a same sense cancellation pulse 210 and an opposite sense cancellation pulse 220. In both cancellation pulse styles, the small droplet pulse occurs at the end of the waveform directly in front of the cancellation pulse 210 or 220. These waveforms have the advantage that the cancellation pulse effectively controls the meniscus motion. These waveforms have the disadvantage that the small droplet formation is late compared to the formation of the other droplets that use pulses that start earlier. This small droplet arrives at the medium (e.g., paper) later because this droplet is formed late. Typically, the fire pulse amplitude is increased in order to compensate. However, since faster droplets tend not to form single droplets, but instead have a slower droplet formed out of the tail, there are practical limits to this strategy.